Automotive fuel vapor canister typically have a bed of activated carbon granules that substantially fills the canister housing, adsorbing and later releasing to the engine excess fuel vapor that would otherwise have been vented. The volume of the carbon bed may decrease relative to the volume of the canister housing over time. This decrease in the relative volume ratio may occur as the result of environmental expansion of the canister housing itself, or as a result of the carbon granules becoming more densely packed, or both. Either effect may cause the carbon bed to become more loosely packed within the housing. The typical mechanism for maintaining loading pressure in the carbon bed is simply a helical compression spring or springs biased directly between a pressure plate and the bottom end cover of the canister. As the volume ratio decreases, the spring or springs expand to move the pressure plate into the carbon bed to maintain it firmly packed.
The inherent characteristics of the canister housing and carbon bed dictate an axial range of motion for the pressure plate, the distance it would have to move in order to stay with the carbon bed. Compression springs have to expand axially over the same range of motion in order to stay with the plate, and have to remain compressed in order to maintain a loading pressure on the carbon bed. Applying a force through a distance means that energy must be stored and released, and a compression spring stores energy by virtue of its axial compression. Equal energy can be provided by compressing longer, weaker springs to a greater degree, or by compressing shorter, stiffer springs to a lesser degree. Limited axial space below the pressure plate may dictate shorter, stiffer springs. However, it is also desirable that the loading pressure provided by the springs stay relatively constant, and compression springs with lower spring rates generally provide a more linear response over a given range of motion. But weaker springs may not store enough energy when compressed within the limited axial space available. Therefore, it may not be possible to optimize all parameters with the conventional, direct acting compression spring design.